1. Field of the Invention
This invention relates broadly to methods and apparatus for performing a heart reshaping intervention. More particularly, this invention relates to methods and apparatus for minimally invasive restoration of the left ventricle in patients suffering from congestive heart failure.
2. State of the Art
In the U.S., approximately 5 million patients are currently diagnosed with congestive heart failure (CHF). CHF generally relates to a dysfunction of the left ventricle. About one third of the patients suffering from CHF have a form of CHF which results from a myocardial infarction (MI). The MI progressively increases the residual volume of blood in the left ventricle, due to stagnation from decreasing contractility of the heart muscle.
The increase in blood volume also results in an increase in left ventricular pressure which increases stress on the wall of the left ventricle. The stress requires the myocardium to work harder which increases oxygen demand. Since oxygen delivery to the heart has already been reduced because of coronary artery disease, the MI and the resulting reduced ventricular output, heart muscle tissue dies and the ventricle expands. This causes the myocardium to stretch, thin out and distend, further decreasing heart performance, decreasing the thickness of the ventricle wall and increasing wall stress.
FIG. 1 shows a normal heart 10 having right ventricle 12, left ventricle 14, right atrium 16 and left atrium 18. Though not illustrated, those skilled in the art will appreciate that there are a pair of valves between each ventricle and its associated atrium. The ventricles are separated by an inter-ventricular septum 20. The left ventricle 14 has what is called a generally elliptical (ellipsoidal) shape.
FIG. 2 shows a heart 10′ suffering from CHF. The left ventricle 14′ is enlarged and assumes a circular (spherical) shape. The stress on the ventricle wall is determined by the Laplace Law as illustrated in Equation 1, below.
                              wall          ⁢                                          ⁢          stress                =                                            (                              pressure                ⁢                                                                  ⁢                in                ⁢                                                                  ⁢                cavity                            )                        ·                          (                              radius                ⁢                                                                  ⁢                of                ⁢                                                                  ⁢                cavity                            )                                            2            ·                          (                              wall                ⁢                                                                  ⁢                thickness                            )                                                          (        1        )            
Thus, as wall thickness is decreased, wall stress increases. This increased wall stress and oxygen demand cause a relative chronic myocardial ischemic state which results in decreased pump function.
It has also been discovered that the change in the shape of the left ventricle adversely affects the way the heart muscle fibers work. The normal ellipsoidal shape most efficiently assists in blood flow through the left ventricle.
State of the art methods for treating CHF involve extremely invasive open heart surgery. For example, use of a “ventricular restoration patch” installed via “purse string” sutures is disclosed in U.S. Pat. No. 6,544,167. The patch seals off a portion of the ventricle thereby reducing the volume and restoring the shape of the cavity. However, installation of the patch requires incision into the left ventricle which severs muscle fibers and the subsequent healing scar increases the risk of arrhythmia.
Another method described in U.S. Pat. No. 6,126,590 involves wrapping the heart in a mesh and suturing the mesh to the heart. The mesh constricts both right and left ventricles, thus not allowing them to fill completely in diastole. It also may cause a constrictive effect on the ventricles known as the tamponade effect.
Yet another method for treating CHF is described in U.S. Pat. No. 6,537,198 and involves the use of trans-ventricular wires anchored by external fixation buttons on either side of the left ventricle. This method puts a compressive force on the ventricle but also results in a mid-level constriction without favorably altering volume, pressure, or wall stress.
Because of the highly invasive nature of these treatments, many CHF patients are not suitable candidates for the surgery.